Solar collector

ABSTRACT

A low profile solar collector collects solar energy as heat, while limiting possible peak stagnant temperatures in the collector in summer months. 
     The collector has a plurality of reflectors, each having longitudinal edges parallel to an adjacent reflector such that the reflective elements are arranged in a fixed offset parallel array, which forms a staircase of upwardly facing reflective surfaces in use. 
     Absorber elements are located between adjacent pairs of reflectors. The absorber elements each have an exposed surface located between each pair of reflective elements, and arranged to extend between a first edge of one reflector and a second edge of an adjacent reflector. The exposed surfaces of the absorber elements are disposed at an angle to each of the reflective elements to thereby form a fixed parallel array of exposed surfaces of the absorber elements. 
     A transparent top cover extends over the reflective and absorber elements. 
     In use the reflective elements are oriented in a first generally horizontal orientation such that incident sunlight passing through the top cover either falls directly onto the exposed surface of each absorber element or the incident sunlight falls a reflector adjacent to an exposed surface of one of the absorber elements and reflects onto the respective exposed surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Australian Provisional Patent Application No 2007900662 filed on 12 Feb. 2007, and Australian Provisional Patent Application No 2007901492 filed on 21 Mar. 2007, the contents of which are incorporated herein by reference.

The present invention relates to the use of solar collectors to collect energy in the form of heat from solar radiation for uses such as space heating and hot water supply for residential, commercial or industrial buildings.

Presently, there is growing concern about the global warming effects caused by the burning of fossil fuels, such as for example in fueling space heaters and domestic hot water systems. The increasing environmental costs associated with such effects are expected to force governments around the world to eventually introduce taxes on the burning of fossil fuels or carbon trading schemes, both of which would increase the cost associated with the burning of fossil fuels.

The use of incident solar radiation for the above mentioned applications is an obvious and fitting alternative to burning fossil fuels and yet presently, such systems are not used extensively. Glass/metal solar hot water system have found some use, but yet even these systems are mostly subsidized by government rebates due to their high up-front costs. The average pay-back time for such systems can be 5 to 10 years and yet glass/metal collectors have an average lifetime of only around 15 years. Needless to say the recent spike in metal prices has not help the economic viability of such systems, since they make extensive use of copper and aluminum.

There have been numerous efforts to use plastic in the production of solar collectors due to lower material costs, yet for a variety of reasons plastic collectors have not been widely commercially produced or are not commercially competitive with glass/metal. These are mainly due to well known limitations of plastic when used for glazing and especially absorbers. The main limitation concerns the exposure of absorber components, or other structural components of the collector to extreme temperatures under stagnant conditions, where the collector is exposed to full sun conditions and where the internal cooling fluid does not flow. Inner stagnant temperature can easily reach upwards of 160° C. for flat solar collectors which have selective surface coatings to maximize absorption while minimizing heat loss. Panel exposure to stagnant conditions is common and can take place during installation or pump, controller or value failure and can easily cause catastrophic damage to plastic collectors or severely limit the collector's life time. This can mandate the use of performance plastics for absorbers and associated components which can withstand these high temperatures but unfortunately such performance plastics are much more costly.

There have been many attempts at limiting stagnant temperatures in plastic solar collectors. The simplest of these allow significant heat loss from the top face of the panel through the use of non-selective coatings for the absorber and glazing or the outright lack of a glazing cover as in pool type solar collector, or the lack of underside insulation. Unfortunately, all of these limit the efficiencies of such solar panels and hence limit their use specifically in winter. More sophisticated passive methods include the use of materials which change their transmission of light as a function of temperature, but these have not found use as solar collector components. Many active control methods can and have been envisaged, such as for example ventilation systems or the mechanical turning of reflector components so that the sun is rejected back out into the atmosphere instead of onto the designated absorber. Such strategies have the obvious disadvantage of using complex moving parts and associated monitoring and controller systems which are prone to high maintenance costs and eventual failure.

SUMMARY OF THE INVENTION

The present invention provides a low profile solar collector, for collecting solar energy as heat, while limiting possible peak stagnant temperatures in the collector in summer months, comprising:

(a) a plurality of reflective elements, having respectively first and second edges, the reflective elements being arranged in a fixed offset parallel array, to form a staircase of upwardly facing reflective surfaces in use;

(b) an absorber element having an exposed surface located between each pair of reflective elements the exposed surface being located substantially between the first edge of one of the adjacent pair of reflective elements and the second edge of the other of the pair of reflective elements, and the exposed surfaces being disposed at an angle to each of the reflective elements to thereby form a fixed parallel array of exposed surfaces of the absorber elements; and

(c) a transparent top cover extending over the reflective and absorber elements;

such that in use the reflective elements are oriented in a first generally horizontal orientation with incident sunlight passing through the top cover and falling directly onto the exposed surface of each absorber element and/or onto a reflector adjacent to an exposed surface of one of the absorber elements and from which it may be reflected onto the respective exposed surface.

Typically the collector panel will be mounted on a flat supporting structure, such as a roof, preferably facing the solar noon sun, where the roof has a pitch p, such that the upper edges of the absorber elements describe a line at an angle p to the horizontal (see FIG. 2 for a definition). The collector may typically be installed on a roof having a pitch of 70° or less but a pitch in the range of 10°-50° is preferred. Insulation may be installed under the absorber structure to reduce heat transmission through to the roof.

The solar collector will preferably include a housing of which the top cover forms part, such that the housing encompasses the array of reflectors and absorber elements.

The sun exposed surface area of the absorber elements (which are not necessarily flat) are preferably angled such that a normal of the sun exposed surface area (or average normal if the sun exposed surface is not flat) makes a larger incident angle with the solar noon sun radiation in midsummer than in midwinter or mid spring/autumn (see FIG. 1-3). Preferably the reflective elements are permanently fixed at a small angle (or average angle if not flat) above or below horizontal.

The reflective elements may be made from materials including, (but not limited to) polished sheet metal which can be placed on or attached to the connecting absorber elements, polycarbonate mirror which can be attached to the glazing or absorber elements, a layer of vacuuming chroming or plating deposited directly on certain areas of the absorber elements, particularly if those areas are substantially flat, or by the addition of a thin sheet of reflective plastic like aluminized Mylar® to the absorber element areas, whereby an aperture is defined between each adjacent pair of reflective elements, the aperture being defined by the first edge of one of the pair of elements and the second edge of the other of the pair of elements.

Preferably, the exposed absorber element surface is blackened and hence has a high solar absorptivity.

Embodiments of the solar collector panel may include one or more transparent top covers which may be formed of a polymer material or glass. Preferably the material is a low cost material, such as a low cost sheet of transparent polymer material or a low cost glass. The top cover material is preferably clear but may partially or completely block ultra violet light transmission. The solar collector may include a plurality of top covers separated by sealed air gaps to reduce heat transfer through the top covers.

In embodiments of the solar collector the fixed parallel array of absorber elements are of an elongate rectangular shape and run the length of one horizontal dimension of the collector. When the solar collector is mounted in an in use position, the orientation of the solar collector is preferably such that these plastic absorber elements extend in a substantially East-West direction. The plastic absorber structure can be manufactured as a single piece, containing these joined absorber elements, or can alternatively be manufactured as separate absorber elements and then arranged or connected in the parallel array.

The fixed parallel array of rectangular reflective elements may be substantially of elongate rectangular shape and also run the length of a horizontal dimension of the collector (the same horizontal dimension as the plastic absorber elements). Therefore again the reflective elements will preferably extend in a substantially East-West direction in use, where reflective elements and absorber elements are arranged to alternate in the assembly; with the reflective elements forming a substantially step-like appearance as viewed in the direction down the pitch of the roof with the reflective elements forming the treads and the absorber elements forming the risers of the steps.

In one embodiment the collector may be mounted such that the lengths of the absorber and reflector elements are at a slight angle to the horizontal in the range of 1°-5°.

Preferably the long edge of one reflector and the long edge of a second reflector form an elongated planar rectangular aperture through which an absorber area or face is exposed to direct and reflected solar insolation, and where this exposed area or face is preferably flat and preferably coincides with the planar rectangular aperture and hence can protrude slightly forward of or be slightly behind the planar rectangular aperture. The planar rectangular aperture and the exposed area or face is also preferably tilted downward toward the horizon at an angle so that the direct midsummer solar noon sun strikes the area or face (or penetrates the rectangular aperture) with a large incident angle, while the direct midwinter or midspring/autumn (equinox) solar noon sun strikes the area or face at a smaller incident angle;

Each sun exposed surface area or face has in front of it (as viewed down the pitch of the roof or supporting structure) a corresponding reflector which is preferably fixed at a small angle, r rotated toward or away from the corresponding sun exposed surface area, the purpose of the corresponding reflector is to reflect winter solar insolation onto the corresponding sun exposed surface area of the absorber element;

The angle, r of each such corresponding reflector is dependent on a roof pitch, p (assuming the collector panel is mounted parallel to the pitch of a roof) and the latitude at the place of installation. The angle r for a particular roof pitch and latitude may be in a range having limits defined by Equation 1 and Equation 2 below. The values r and p are preferably chosen such that the majority of the winter solar radiation impinging upon the reflective elements is reflected upon the corresponding sun exposed surface area, for the purposes of concentrating wintertime radiation, while rejecting back out into the atmosphere all of, or some portion of the summer solar radiation, for the purposes of limiting summertime stagnant temperatures.

The absorber structure has at least one fluid channel underneath the sun exposed surfaces in thermal communication with the exposed surface of the absorber elements and optionally underneath the reflector areas. These channels may run the length of the panel (parallel with the adjacent edges of the reflectors and absorber elements) or alternatively they may run perpendicularly to the reflectors (up the pitch of the roof) or at any angle. Preferably each absorber element has at least one internal fluid carrying channel extending underneath the exposed surface thereof and—in thermal communication with the exposed surface to carry heat away to be stored or otherwise used. These channels can also run the length of the panel (parallel with the reflective elements and absorber elements) or alternatively perpendicular to the reflective elements or at any such angle.

When the fluid carrying channels are such that they also run underneath the reflector areas then it may be advantageous in certain situations to additionally allow significant thermal communication between the reflector areas and the fluid carrying channels below such that any residual heat on the reflector areas may is also transferred to the fluid.

The collector includes an inlet passage to introduce the heat transfer fluid into the collector panel and the fluid channels and an outlet passage to drain the heat transfer fluid from the channels and the collector panel.

Preferably the absorber element (incorporating the fluid channels) is to be formed out of a polymer material such as for example a plastic material.

The plastic absorber elements may be UV stabilized and/or painted or treated with a selective coating on the sun exposed surface areas or left untreated. The absorber elements may also have a thin surface selective film attached to the sun exposed surface areas. The glazing may also be UV stabilized and made to block UV radiation and may also be treated with a non-reflective or selective coating.

In one preferred embodiment, the top glazing cover and the absorber assembly are connected by sliding engagements comprising an undercut channel and a co-operating ribbed projection having a complementary cross sectional shape to that of the channel. In this arrangement, each absorber element includes one half of the engagement namely one of the projection or the channel and an adjacent surface of the top glazing cover is provided with the respective co-operating part which forms the other half of the engagement. Preferably each absorber element will include an undercut channel and the adjacent surfaces of the top glazing cover are provided with the respective co-operating ribbed projections.

Preferred embodiments may also include a bottom cover connected to the absorber assembly by sliding engagements similarly comprising an undercut channel and a co-operating ribbed projection having a complementary cross sectional shape to that of the channel. In this arrangement, each absorber element again includes one half of the engagement namely one of the projection or the channel and an adjacent surface of the bottom cover is provided with the respective co-operating part which forms the other half of the engagement. Preferably each absorber element will include an undercut channel and the adjacent surfaces of the bottom cover are provided with the respective co-operating ribbed projections.

The absorber elements of the solar collector may have a midsummer solar noon sun acceptance aperture (see FIG. 1-3 for definition) of between than 50% and 0%, while at the same time having a midwinter solar noon sun acceptance aperture of approximately 100%. Preferably the absolute mid winter solar noon sun acceptance aperture is at least 2 times the midsummer solar noon sun acceptance aperture.

The reflective elements may also reduce heat loss from the absorber element fluid channel or channels which can run substantially underneath the reflective areas. To this end the reflectors may be made from a material with a very low emissivity.

The possible angles of the stepped reflective elements are determined by the pitch of the roof and the latitude of the place of installation and may be, for any particular pitch and latitude, in the range of values between those given by the equations 1 & 2 below:

$\begin{matrix} {r = \frac{\left( {w - 11.75} \right) - p}{2}} & (1) \\ {r = \frac{\left( {w + 23.5} \right) - p}{2}} & (2) \end{matrix}$

Where r is the angle of the reflective element, w is the angle of the solar noon midwinter sun at the particular place of installation and p is the pitch of the roof (see FIG. 1 for all angle definitions).

Equation 1 gives the lower limit for the reflector angle for systems which are substantially winter only collectors and which may need to be installed on roofs which are oriented slightly off solar noon (i.e. which need to maximize the capture of lower winter sun angles). Such applications do not require substantial summertime collection.

Equation 2 gives the upper limit for the reflector angles where maximum heat collection takes place near the Spring and Autumn equinox points. However, the embodiment still entails significant summertime rejection and hence protection against summertime stagnant temperatures. Reflector angles near the upper range correspond to installation conditions where a substantial amount of heat is still required in the summertime, such as for domestic hot water systems only.

The preferred reflector angle for installations where the collector faces the solar noon sun is that angle where the midwinter solar noon sun hits the outer edge of the reflector and is reflected to the top edge of the absorber face. The equation for this value of r is:

r=(w−p)/2   (3)

Embodiments of the solar collector may make use of surface selective coatings and insulation underneath the absorber structure while still maintaining summer time stagnant temperatures below destructive levels such that the use of inexpensive non-performance plastics is acceptable.

Where a solar collector covers a substantial part of the solar noon facing roof it may also help to lower roof cavity temperatures and hence internal house temperatures in summer by providing a good form of reflective and non-reflective insulation on the roof.

A preferred solar collector may collect more energy in winter or a relatively constant supply of heat energy in the four seasons and it may thereby avoid overheating of storage tank water and hence avoid the need for an active heat preventative provision in the summer. Such solar collectors may also provide a flatter heat collection curve throughout the day, eliminating sharp collection peaks especially in the summer months.

Although some plastics may be less robust materials, a plastic collector may still provide a longer lasting collector. Such collectors, for example, may not suffer from problems including internal corrosion or clogging with deposits from use of hard water, or damaged due to freezing of water inside inflexible metal piping, as do metal collectors. Because the top glazing cover may need periodic replacement solar collectors may have a provision for the easy replacement of this plastic cover which substantially covers the entire collector and which blocks a substantial amount of UV radiation.

Solar collectors may be sufficiently light that a slim-line roof supported collector may take advantage of a typical roof as an existing flat supporting structure and hence to obviate the need for any extensive internal or external collector supporting or stiffening structures. Such a collector might use a minimum of materials in construction resulting in a lower cost of production.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of a solar collector will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 shows a solar collector fitted to a roof structure.

FIG. 2 schematically illustrates a cross sectional view of the top part of a simple stepped absorber/reflector structure;

FIG. 3 schematically illustrates a cross sectional view of a structure of a solar collector showing a small curvature in the reflector element;

FIG. 4 schematically illustrates a solar collector suitable in situations where less winter heating is required;

FIG. 5 schematically illustrates another solar collector arrangement suitable in situations where less winter heating is required;

FIG. 6 schematically illustrates a solar collector, showing a possible arrangement of absorber fluid channels in the absorber elements;

FIG. 7 schematically illustrates a solar collector showing a possible arrangement of a plurality of absorber fluid channels in each absorber element;

FIG. 8 schematically illustrates a solar collector showing a method for the attachment of a top glazing cover and a bottom housing cover;

FIG. 9 schematically illustrates a solar collector similar to that of FIG. 8 with a different modulation for the top glazing cover;

FIG. 10 schematically illustrates a solar collector similar to that of FIG. 8 with yet another top glazing cover fitted;

FIG. 11 schematically illustrates a solar collector with a triangular fluid channel in each absorber element;

FIG. 12 schematically illustrates a solar collector with a rectangular fluid channel in each absorber element;

FIG. 13 schematically illustrates a solar collector with a rectangular fluid channel in each absorber element and where the array of absorber elements is constructed by joining absorber elements directly to each other;

FIG. 14 schematically illustrates a solar collector with a cylindrical sun exposed surface area and fluid channel for each absorber element, and positioned partly under the respective reflective element;

FIG. 15 schematically illustrates a solar collector with a plurality of fluid channels running perpendicularly to the sun exposed areas of the absorber elements; and.

FIG. 16 schematically illustrates a solar collector in which each fluid channel services a plurality of absorber elements and reflectors.

DETAILED DESCRIPTION OF EMBODIMENTS OF A SOLAR COLLECTOR PANEL

Referring to FIG. 1 a solar collector 11 is shown fitted to a roof structure 19. Such an installation may be made as a retrofit or may be completed at the time of building in which case the roof may be modified specifically to allow simplified installation and ducting of pipe work. The embodiment shows a solar collector 11 with a typical casing structure including side walls 13, top cover 14, and a bottom cover or wall 26 (ref FIGS. 6-14 & 16), with an internal arrangement of absorber elements 10 and reflectors 12 configured in a step-like structure. Inlet 15 and outlet 17 end piping connections are provided for the absorber fluid channels 22 (ref FIGS. 6-15). Piping (not shown) connected to the inlet 15 and outlet 17 will be lead from the roof 19 and connected to a heat storage system or used directly for the provision of useful heat.

The preferred solar collector 11 is a low profile panel which absorbs solar energy and converts it to heat. A passive or non-mechanical method is used for limiting peak stagnant temperatures in the collector in the summer months such that low cost materials may be employed which could otherwise potentially be destroyed if the peak stagnant temperature was not limited.

The collector panel 11 is substantially flat and has major components such as absorber elements 10 and top glazing cover formed of plastic or other low cost materials.

The preferred solar collector is proposed to be mounted on a flat supporting structure, such as a roof, having a pitch of up to 60° but preferably between 10° and 50° measured from the horizontal. The solar collector 11 is preferably mounted to face the direction of the solar noon sun (i.e. due North in the Southern Hemisphere or due South in the Northern Hemisphere), but may be mounted at a small angle to one side of the direction of the solar noon sun without significant loss of efficiency.

Referring to FIGS. 2 to 5, the panel comprises a fixed parallel array of elongate plastic absorber elements 10 with sun exposed areas 21 each of which run the horizontal length of the collector. In use the elements will preferably extend in a substantially East-West direction. A fixed parallel array of substantially flat elongate rectangular reflective elements 12, are also spaced apart in parallel arrangement with lengths which also run the horizontal length of the collector, substantially in an East-West direction. The lengths of reflective elements and absorber elements are arranged in an alternating absorber/reflector fashion; with the reflective elements forming a substantially step-like appearance as viewed in the direction down the pitch of the roof.

Cross-sectional drawings of the structures of various embodiments are shown by way of example in FIG. 6-16. The complete absorber structure 28 can be manufactured as a single extruded piece, comprising absorber elements 10 joined by connecting pieces where the reflectors will be located. Alternatively this may be achieved, for example by (concertina) cutting and folding a single piece of multiwall material to form a stepped top surface of an absorber structure such as that illustrated in FIG. 15. Reflective material may be subsequently applied to the connecting pieces. Alternatively the separate absorber elements 10 and reflectors 12 may be assembled and connected to form the parallel array.

At least one transparent (clear) plastic glazing top cover 14 is located over the absorber structure 28 to retain heat and protect the absorber structure.

The long edges of pairs of adjacent reflectors form an elongated planar rectangular aperture through or within which an absorber area or face 21 of an absorber element 10 is exposed to direct and reflected solar insolation. This exposed area or face 21 is preferably flat and preferably coincides with the planar rectangular aperture and hence can protrude slightly forward of or be slightly behind the planar rectangular aperture. As seen in FIGS. 2-5, the planar rectangular aperture and the exposed area or face is substantially vertical or at least at an angle to the horizontal so that the direct midsummer solar noon sun strikes the area or face (or penetrates the rectangular aperture) with a large incident angle, while the direct midwinter or midspring/autumn (equinox) solar noon sun strikes the area or face at a smaller incident angle. A corresponding reflector 12 is located in front of each sun exposed surface area or face 21 (as viewed down the pitch of the roof or supporting structure) which is rotated toward or away from the corresponding sun exposed surface area and fixed at a small angle r which may be positive or negative with respect to the horizontal (see FIG. 2). The reflective elements each function to reflect winter solar insolation onto the sun exposed surface area 21 of the corresponding adjacent absorber element 10.

The angle, r of the reflective elements is dependent on the roof pitch, p (refer again to FIG. 2 ) and the latitude at the place of installation, and for a particular roof pitch and latitude this angle can be in the range of values between those given by Equation 1 and Equation 2 (above). The values of r and p should be chosen in such as way that the reflective elements reflect the majority of the winter solar radiation impinging upon the reflector onto the corresponding sun exposed surface areas, for the purposes of concentrating wintertime radiation, while rejecting back out into the atmosphere all of, or some portion of the summer solar radiation, for the purposes of limiting summertime stagnant temperatures.

While the solar collector can be adapted to a variety of heat absorber technologies, in the preferred embodiment each absorber element has at least one inner fluid carrying channel 22 to which heat is transferred and carried away to be stored, and where these channels run underneath the sun exposed absorber areas 10 and can also optionally run underneath the reflective areas (see FIGS. 11 and 15 for example). These channels can run the length of the panel (parallel with the reflective elements and absorber elements) as shown in FIGS. 6-14 & 16 or alternatively perpendicular to the reflective elements (up the pitch of the roof, see FIG. 15) or at any such angle.

Proposed embodiments of the solar collector at first may seem counterintuitive since the collector efficiency is deliberately limited during the best solar collecting season namely, summer, while nearly all previous solutions seek to increase efficiencies for every season. Furthermore, with the present collector nearly twice the panel area is needed to collect the same amount of energy over an entire year as a standard flat solar panel will collect over a year, hence this may also seem counterintuitive. For the present solar collector it must be realized that firstly, for typical situations the need for hot water in wintertime is much greater than the need in summertime and hence if we had a large enough panel installation to supply sufficient wintertime hot water (for domestic heating and the hot water service) then we would have much more than enough hot water in summer and hence we can easily sacrifice summertime efficiency without approaching or dropping under the maximum summer hot water requirement. Secondly, it must be realized that a typical hot water solar flat panel installation today takes up much less roof area than is available, and hence a typical roof area can accommodate a much larger panel. Thirdly, most prior art units focus on maximizing heat collected per square meter, while it must be realized that the sun's energy per square meter is of course free, and while the available roof area (or supporting structure) for a typical house/building is generally already existent and of a size that could accommodate larger collectors that commonly in use today. Therefore there is no significant extra cost associated with supporting a larger panel or more panels, hence the design of the present solar collector focuses on cost per unit of heat collected, making up the shortfall in heat collection over the entire year by increasing the area of the collector. By employing a much cheaper method of construction for the solar collector, this approach makes possible a significantly lower cost per unit of heat.

Preferred collector arrangements of this type can potentially produce a solar panel with a midsummer solar noon sun acceptance aperture (refer to FIGS. 1-3) (direct and reflected), of approximately between than 50% and 0%, while at the same time having a midwinter solar noon sun acceptance aperture (direct and reflected) of approximately 100%, and where the absolute midwinter solar noon sun acceptance aperture is at least 2 times that of the midsummer solar noon sun acceptance aperture.

Hence, it becomes possible to produce a passive solar panel of this type with the dual benefit of preventing overheating in the summertime while concentrating wintertime insolation. While the proposed panel does significantly concentrate wintertime insolation and hence could run into winter time stagnant temperature issues, it must be realized that for the majority of places on earth, the wintertime temperatures are much lower than summertime temperatures and the winter sun's radiation is lower in the sky and as such has more atmosphere to go through and hence is of a lower concentration than the summer's sun, and thirdly for the envisaged typical installation (for a roof pitch around 25-35°) the winter's sun strikes the entire collector at a higher incident angle than during other seasons. As such this collector for the first time allows the use of glazing (or double glazing) with effective surface selective coatings and very good underside insulation in combination with the use of inexpensive non-performance plastics in the majority of the structure without any concerns over summertime stagnant temperatures. This has the potential to provide a solar panel which is very efficient in the wintertime while purposefully being less efficient during summer, at a much lower cost than traditional flat plate collectors.

Using the principles outlined above it becomes possible to construct a solar collector which can collect more energy in winter or a relatively constant supply of heat energy in the four seasons. As an added advantage the collector may avoid overheating of storage tank water and hence the need for an active heat preventative provision in the summer, and thereby potentially lowering total system costs.

It also becomes possible to provide a flatter heat collection curve throughout the day, eliminating sharp collection peaks especially in the summer months when this is a more commonly a problem.

The panel may also be constructed to serve the purpose of providing a good form of reflective and non-reflective insulation on the roof and hence help to lower roof cavity temperatures and hence internal house temperatures in the summer, where the solar collector covers a substantial part of the solar noon facing roof.

By use of plastic components it also becomes possible to construct a collector panel which is cheaper and much longer lasting and which does not corrode or internally clog with deposits from use of hard water, or become damaged due to freezing of water inside inflexible metal piping as do metal collectors. Toward this end it is likely that the top glazing cover may need periodic replacement. For this reason it is proposed to make provision in preferred embodiments for the easy replacement of this plastic cover, which substantially covers the entire collector and which can block a substantial amount of UV radiation.

By constructing a slim-line roof supported panel it may also be possible for the collector to take advantage of a typical roof as an existing flat supporting structure and hence to obviate the need for any extensive internal or external collector supporting or stiffening structures and as such to provide a collector which uses a minimum of materials in construction and which substantially lowers the cost of the collector and its installation.

Referring to FIG. 2 a cross sectional schematic of a typical example of the top part of the stepped structure of the absorbers 10 and reflectors 12 is illustrated. The example in FIG. 2 shows the reflective elements 12 and sun exposed absorption areas or faces 21 of the absorber elements 10 and their typical step-like relation to each other. FIG. 1 also shows the overall acceptance aperture 16S and rejection aperture 18S for the midsummer solar noon sun altitude 42, using a latitude of 37.5° as an example (giving a midwinter solar noon sun elevation as 29°). The mid autumn solar noon sun altitude 41 in FIG. 1 is shown to provide an overall acceptance aperture 16A and rejection aperture 18A while the midwinter solar noon sun altitude 40 is shown to provide an overall acceptance aperture 16W and the rejection aperture has disappeared. The given configuration happens to show about 75% rejection of midsummer solar noon insolation, of course other applications with various summertime rejection percentages are also easily possible by varying, r and the angle of the sun exposed absorption area 21 as shown below. The angle of the reflective element, r (which happens to be near horizontal for this example application) is measured counterclockwise from the horizontal. The elevation angle w of the solar noon midwinter solar radiation 40, is also measured counterclockwise from the horizontal, while the pitch of the roof or supporting structure on which the collector is installed, p is measured clockwise from the horizontal. In the figure the planar rectangular aperture formed by the edges of two opposing reflective elements is shown to coincide with the sun exposed surface area 21 of the absorber element 10.

FIG. 3 shows the cross sectional structure of another embodiment of the collector showing a small curvature in the reflector element. Such an embodiment is useful where a high level of heat is required in the wintertime while significantly less heat is required in the summertime.

FIG. 4 shows another embodiment of a solar collector where less winter heating is required, and as such a greater fraction of a smaller roof panel is used to provide hot water in the summer months. The figure shows approximately 50% rejection of midsummer solar noon insolation. The sun exposed surface area 10 for such an application is shown to be angled more toward the Midspring-Autumn sun. In FIG. 4 the planar rectangular aperture formed by the edges of two adjacent reflective elements 12 is shown to coincide with the sun exposed surface area of the absorber element 10.

FIG. 5 shows another embodiment of a solar collector where less winter heating is required, and as such a greater fraction of a smaller roof panel is used to provide hot water in the summer months, but showing slightly higher reflector angle r. The figure shows approximately 60% rejection of midsummer solar noon insolation. In the figure the planar rectangular aperture formed by the edges of two opposing reflectors is shown to coincide with the sun exposed surface area of the absorber element 10.

FIG. 6 shows another embodiment of a solar collector with near cylindrical absorber fluid channels 22 where the absorber structure 28 (comprising a plurality of absorber elements 10 and reflectors 12) is made as a single (e.g. extruded) piece of plastic with the reflective elements 12 effectively supported by a flat connecting regions 48 between the absorber elements 10. The figure also shows an optional secondary glazing cover 20 for the radiation receiving absorber surface area 21 being attached to the top glazing cover 14. The figure also shows optional insulation 24 and a bottom cover 26. In the figure the planar rectangular aperture formed by the edges of two opposing reflective elements is shown not to coincide with the sun exposed surface area 21 of the respective absorber element 10 which is substantially curved and forward of, and hence is seen to protrude out from the rectangular aperture.

If the fluid medium consists of air, then the air can run preferably through the parallel arrangement of substantially east-west running channels 22 and the channels 43 in front of the sun exposed surface absorber areas 21. Initially the air can enter the collector through the first channels 43 in front of the absorber face/reflector structure, then through the absorber sun exposed face 21 (which is in this instance is porous to air) and then through the second set of channels 22 underneath the heat absorbing surfaces in the same direction as the incoming air and exiting on the opposite side of the collector. Alternatively, air can enter at the bottom of the collector, but in the front section of the absorber/reflector structure, again moving through the absorber sun exposed face 21 (which is porous to air) and exit out of the collector from the underneath absorber/reflector structure at the top of the collector.

FIG. 7 shows another embodiment of a solar collector with a plurality of absorber fluid channels 22 where the complete absorber structure 28 is made as a single (e.g. extruded) piece containing the absorber elements 10 and the radiation receiving absorber area 21 is flat, with the reflective elements 12 effectively supported by a flat region 48 on between adjacent absorber elements 10. The figure also shows optional insulation 24 and a bottom cover 26. In the figure the planar rectangular aperture formed by the edges of two opposing reflective elements is shown to coincide with the sun exposed surface area 21 of the absorber element 10.

The examples of solar collector panels illustrated in FIGS. 8, 9 & 10 show a means for attaching the top glazing cover 32 which includes modulations of various shapes, as well as bottom covers 26, to the step reflector/absorber structure 28. In this embodiment the figure shows a system of attachment comprising a slide engaging in a channel. Open undercut channels 44 on upper edges of the absorber elements 10 receive projecting ribs 45 extending from the bottom surface of the top cover 32, and which slide into engagements from the side of the absorber structure 28 to connect the top cover 32 to the absorber structure 28. The embodiment also show three variations of top cover with different modulation patterns in the top glazing cover 32 of FIGS. 8 & 9 and a flat top cover in FIG. 10. The modulation prevents bucking of the top glazing cover due to thermal expansion of collector elements during operation. In the case of the bottom cover 26, a similar arrangement of channels 46 and ribs 47 permit the bottom cover to be slid onto the absorber structure 28. A reversal in the male/female aspect of the attachment system is also possible, in which case the absorber structure 28 would carry the male projecting ribs and the glazing cover and bottom covers would carry the female open undercut channels.

FIG. 11 shows another embodiment of a solar collector 11 with a single triangular fluid channel 22 per absorber element 10, with the fluid channel positioned substantially under the reflective elements and where the complete absorber structure 28, comprising the absorber elements 10, is made as a single piece and the radiation receiving absorber area 21 is flat, with the reflective elements 12 effectively supported by a flat region 48 bridging the absorber elements 10. The figure also shows an optional secondary glazing cover 20 for the radiation receiving absorber surface area 21 being attached to the top glazing cover 14. The planar rectangular aperture formed by the edges of two opposing reflective elements 12 is shown to coincide with the sun exposed surface area 21 of the absorber element 10.

FIG. 12 shows another embodiment of a solar collector with a single rectangular fluid channel 22 per absorber elements 10 where the absorber as a whole is constructed by joining absorber elements 10 and the radiation receiving absorber area 21 is L shaped, with the reflective elements 12 effectively being polished sheet metal (or other material), which also serves the purpose of joining the absorber elements 10. In the figure the planar rectangular aperture formed by the edges of two opposing reflective elements 12 does not in this case coincide with the sun exposed surface area of the absorber element 10 which is substantially L shaped and hence is substantially forward of the rectangular aperture. The absorber elements 10 are provided with longitudinal slots 49 in rear top and front bottom edges into which reflectors 12 are slotted to join the absorber elements together in a stepped array.

FIG. 13 shows another embodiment of a solar collector with a single rectangular fluid channel 22 per absorber element 10 where the array of absorber elements as a whole is constructed by joining absorber elements directly to each other. In this case each absorber element is formed with an integral reflector and the array is assembled by inserting a rear end of each reflector 12 into a slot 49 in the lower front edge of the next absorber element in the array 28, as shown. The radiation receiving absorber area 21 is flat in this example, with reflective elements 12 being effectively supported by absorber elements 10. The planar rectangular aperture formed by the edges of two opposing reflective elements 12 is in this case shown to coincide with the sun exposed surface area of the absorber element 10.

FIG. 14 shows another embodiment of a solar collector with a single spherical fluid channel 22 per absorber element, with the fluid channel positioned partly under the reflective elements where the absorber structure 28 is made as a single piece and the absorber elements 10 are cylindrical, with semi-cylindrical radiation receiving faces 21. The reflective elements 12 shown are made from polycarbonate mirror 12 and are attached to the top glazing cover 14. Secondary glazing 20 which substantially covers the sun exposed surface area 21 of the absorber elements 10 is also attached to the top glazing cover 14. The planar rectangular aperture formed by the edges of two opposing reflective elements 12 does not in this case coincide with the sun exposed surface area 21 of the absorber element 10, the sun exposed surface being substantially curved shaped and set back slightly from the rectangular aperture.

FIG. 15 shows another embodiment of a solar collector with a plurality of fluid channels 22 running perpendicular to the long (East-West in use) axis of the exposed surfaces 21 of the absorber elements 10 and reflector elements 12 which form the stepped structure 28. In FIG. 15 the planar rectangular aperture formed by the edges of two opposing reflective elements is shown to coincide with the sun exposed surface area 21 of the absorber element 10.

FIG. 16 shows yet another embodiment of a solar collector with a plurality of (miniaturized) pairs of sun exposed absorber faces 21 and reflector elements 12 per fluid channels 22. The fluid channels 22 run parallel with the absorber elements 10 and reflector 12.

It will be appreciated by persons skilled in the art that numerous variations, combinations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

1. A low profile solar collector, for collecting solar energy as heat, while limiting possible peak stagnant temperatures in the collector in summer months, comprising: (a) a plurality of reflectors, having respectively first and second edges, the reflective elements being arranged in a fixed offset parallel array, to form a staircase of upwardly facing reflective surfaces in use; (b) an absorber element having an exposed surface located between each pair of reflective elements the exposed surface being located substantially between the first edge of one of the adjacent pair of reflective elements and the second edge of the other of the pair of reflective elements, and the exposed surfaces being disposed at an angle to each of the reflective elements to thereby form a fixed parallel array of exposed surfaces of the absorber elements; and (c) a transparent top cover extending over the reflective and absorber elements; such that in use the reflective elements are oriented in a first generally horizontal orientation with incident sunlight passing through the top cover and falling directly onto the exposed surface of each absorber element and/or onto a reflector adjacent to an exposed surface of one of the absorber elements and from which it may be reflected onto the respective exposed surface.
 2. The solar collector as claimed in claim 1 wherein the solar collector is arranged to be mounted on a flat supporting structure.
 3. The solar collector as claimed in claim 1 wherein, when mounted in an in use position, the plastic absorber elements extend in a substantially East-West direction.
 4. The solar collector as claimed in claim 1 to wherein all of the parts of the collector are made of plastic.
 5. The solar collector as claimed in claim 1 to wherein the transparent top cover is made of plastic.
 6. The solar collector as claimed in claim 1 wherein the transparent top cover is formed of a polymer material.
 7. The solar collector as claimed in claim 1 wherein the absorber elements are formed of a polymer material.
 8. The solar collector as claimed in claim 1 wherein the transparent top cover is formed of glass.
 9. The solar collector as claimed in claim 1 wherein the collector panel includes a plurality of top covers separated by sealed air gaps.
 10. The solar collector as claimed in claim 1 wherein the absorber elements have at least one fluid channel underneath the exposed surfaces in thermal communication with the exposed surface of the absorber elements.
 11. The solar collector as claimed in claim 1 wherein the absorber elements have at least one fluid channel underneath the reflector areas.
 12. The solar collector as claimed in claim 10 wherein the fluid channel runs the length of the panel parallel with the reflective elements and absorber elements.
 13. The solar collector as claimed in claim 10 wherein the fluid channels run perpendicularly to the reflectors.
 14. The solar collector as claimed in claim 1 wherein each of the absorber elements has at least one internal fluid carrying channel extending underneath the exposed surface and in thermal communication with the exposed surface of the absorber elements.
 15. The solar collector as claimed in claim 14 wherein the fluid carrying channels run underneath the reflective area.
 16. The solar collector as claimed in claim 14 wherein the fluid channels run perpendicular to the reflective elements.
 17. The solar collector as claimed in claim 1 to wherein the collector is installed on a roof having a pitch of 60° or less.
 18. The solar collector as claimed in claim 1 wherein insulation is installed under the absorber structure.
 19. The solar collector as claimed in claim 1 wherein the top cover forms part of a housing which encompasses the array of reflectors and absorber elements
 20. The solar collector as claimed in claim 1 wherein the exposed absorber surface is blackened and has a high solar absorptivity.
 21. The solar collector as claimed in claim 1 wherein the stepped reflective elements are arranged at an angle to horizontal determined by the pitch of the roof and the latitude of the place of installation and, for any particular pitch and latitude, is selected from the range of values between those given by the equations 1 & 2 below: $\begin{matrix} {r = \frac{\left( {w - 11.75} \right) - p}{2}} & (1) \\ {r = \frac{\left( {w + 23.5} \right) - p}{2}} & (2) \end{matrix}$ Where r is the angle of the reflective element, w is the angle of the solar noon midwinter sun at the particular place of installation and p is the pitch of the roof (see FIG. 1 for all angle definitions).
 22. The solar collector as claimed in claim 21 wherein r=(w−p)/2.
 23. The solar collector as claimed in claim 1 wherein the top glazing cover and the absorber assembly are connected by sliding or clip in engagements comprising an undercut channel and a co-operating ribbed projection having a complementary cross sectional shape to that of the channel to engage therein.
 24. The solar collector as claimed in claim 23 wherein and each absorber element includes one of the projection or the channel and an adjacent surface of the top glazing cover is provided with the respective co-operating part of the engagement.
 25. The solar collector as claimed in claim 24 wherein and each absorber element includes an undercut channel and the adjacent surfaces of the top glazing cover are provided with the respective co-operating ribbed projections.
 26. The solar collector as claimed in claim 1 wherein the bottom cover and the absorber assembly are connected by sliding or clip in engagements comprising an undercut channel and a co-operating ribbed projection having a complementary cross sectional shape to that of the channel to engage therein.
 27. The solar collector as claimed in claim 26 wherein and each absorber element includes one of the projection or the channel and an adjacent surface of the bottom cover is provided with the respective co-operating part of the engagement.
 28. The solar collector as claimed in claim 27 wherein and each absorber element includes an undercut channel and the adjacent surfaces of the bottom cover are provided with the respective co-operating ribbed projections. 